Recently, developments of deep oil wells have been promoted. Alloy materials to be used for such deep oil wells are required to have high strength. Moreover, a deep oil well has a high-temperature corrosive environment. Such a high-temperature corrosive environment has a temperature of around 200° C. and contains hydrogen sulfide. In a high-temperature corrosive environment, stress corrosion cracking (SCC) is likely to occur. Therefore, an alloy material for oil well, such as a casing and a tubing to be used for an oil well having a high-temperature corrosive environment, is required to have high strength and excellent SCC resistance.
However, as the strength of an alloy material for oil well increases, the hot workability thereof will deteriorate. Therefore, an alloy material for oil well is required to have excellent hot workability as well as high strength and excellent SCC resistance.
High alloy materials for use in a high-temperature corrosive environment have been disclosed in JP2-14419B (Patent Literature 1), JP63-83248A (Patent Literature 2), JP3650951B (Patent Literature 3), and JP3235383B (Patent Literature 4).
A high-alloy stainless steel disclosed in Patent Literature 1 consists of, in weight %, C: 0.005 to 0.3%, Si: 5% or less, Mn: 8% or less, P: 0.04% or less, Cr: 15 to 35%, Ni: 5 to 40%, N: 0.01 to 0.5%, S: 30 ppm or less, O: 50 ppm or less, one or more kinds of Al and Ti: 0.01 to 0.1%, one or more kinds of Ca and Ce: 0.001 to 0.03%, with the balance being Fe and impurities. In this high-alloy stainless steel, 3(Cr+1.5Si+Mo)−2.8(Ni+0.5Mn+0.5Cu)−84(C+N)−19.8 is −10% or more, and S+O−0.8Ca−0.3Ce is 40 ppm or less. Patent Literature 1 describes that since this high-alloy stainless steel has the above described chemical composition, it has excellent corrosion resistance and hot workability.
A high-Ni alloy for oil well pipe disclosed in Patent Literature 2 consists of, in weight %, C: 0.02% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.01% or less, S: 0.01% or less, Cr: 18 to 28%, Mo: 3.0 to 4.5%, Ni: 18 to 35%, N: 0.08 to 0.20%, Ca: 0 to 0.01%, Mg: 0 to 0.01%, with the balance being Fe and impurities. This high-Ni alloy for oil well pipe has excellent SCC resistance. Further, Patent Literature 2 describes that the hot workability thereof is improved when Ca and/or Mg is contained.
A seamless steel pipe for oil well disclosed in Patent Literature 3 consists of, in weight %, Si: 0.05 to 1%, Mn: 0.1 to 1.5%, Cr: 20 to 35%, Ni: 25 to 50%, Cu: 0.5 to 8%, Mo: 0.01 to 1.5%, sol. Al: 0.01 to 0.3%, N: 0.15% or less, REM: 0 to 0.1%, Y: 0 to 0.2%, Mg: 0 to 0.1%, and Ca: 0 to 0.1%, with the balance being Fe and inevitable impurities. Further, in this seamless steel pipe for oil well, C, P, and S in the impurities are 0.05% or less, 0.03% or less, and 0.01% or less, respectively. This seamless steel pipe for oil well further satisfies Cu≥1.2−0.4(Mo−1.4)2. Patent Literature 3 describes that this seamless steel pipe for oil well has excellent stress corrosion cracking resistance and excellent hot workability.
A high Cr-high Ni alloy disclosed in Patent Literature 4 consists of, in weight %, Si: 0.05 to 1.0%, Mn: 0.1 to 1.5%, Cr: 20.0 to 30.0%, Ni: 20.0 to 40.0%, sol-Al: 0.01 to 0.3%, Cu: 0.5 to 5.0%, REM: 0 to 0.10%, Y: 0 to 0.20%, Mg: 0 to 0.10%, and Ca: 0 to 0.10%, with the balance being Fe and inevitable impurities, wherein C, P, and S in the impurities are 0.05% or less, 0.03% or less, and 0.01% or less, respectively. This high Cr-high Ni alloy has excellent hydrogen sulfide corrosion resistance. Patent Literature 4 describes that the hot workability of this high Cr-high Ni alloy will be further improved when REM, Y, Mg, and Ca are contained.